Secondary batteries such as lithium ion batteries contain a porous polymer membrane impregnated with an electrolytic solution as a separator between a cathode and an anode. The electrolytic solution is volatile and flammable, which is problematical for safety when leakage occurs.
It is known that ammonium salts of certain kinds, such as imidazolium salts and pyridinium salts, become liquid molten salts at 100° C. or lower, particularly around room temperature, and exhibit high ionic conductivity at relatively low temperatures of 200° C. or lower even in the absence of water or an organic solvent. In view of the unique non-volatility of these molten salts, application as an electrolyte of batteries, etc. has been studied. Being liquid, however, they are not easy to handle. In order to facilitate handling of the molten salts, several proposals have been made on a polymer electrolyte having a molten salt immobilized with a polymer.
For example, JP-A-8-245828 discloses a composition comprising an aliphatic quaternary ammonium salt of an organic carboxylic acid and a polymer, such as polyvinyl chloride, polyacrylonitrile or an aliphatic polyether. JP-A-7-118480 discloses a combination of a room-temperature molten salt and a polymer of a vinyl monomer having an alkyl quaternary ammonium salt structure. JP-A-10-83821, JP-A-2000-3620, and JP-A-2000-11753 propose an aliphatic molten salt type polymer synthesized from an imidazolium compound and an acid or an acid monomer. A. Noda, et al., Electrochim Acta, vol. 45, 1265 (2000) and JP-A-11-86632 report a composition of a vinyl polymer and a molten salt. JP-A-10-265673 discloses a polymer composite comprising a non-fluorine polymer and an ionic liquid. Because all these compositions use a polymer the main chain of which is composed mainly of an aliphatic hydrocarbon group, they are inferior in durability characteristics including resistance to oxidation.
JP-A-11-306858 discloses a composition of a vinylidene fluoride polymer and an imidazolium salt, and J. Electrochem. Soc., vol. 147, 34 (2000), Electrochimica Acta, vol. 46, 1703 (2001), and JP-A-11-86632 teach compositions comprising an acid group-containing perfluoropolymer and a molten salt. A composition containing a fluoropolymer is expected to have improved durability but is disadvantageous from the viewpoint of cost and environmental burdens involved in the production of fluoropolymers. It has therefore been demanded to develop an inexpensive and yet durable molten salt composition containing a hydrocarbon polymer.
JP-A-11-86632 proposes a molten salt type polymer electrolyte having a porous polymer solid or a polymer thin film made of a polyanion resin (having negative charges introduced) impregnated with an imidazolium salt derivative. In the practice, however, the proposed technique involves such post-treatments as introduction of a carboxyl group to a porous Teflon membrane using liquid ammonia and sodium or irradiation of a sodium polymethacrylate film with γ-rays to make the film porous. Furthermore, a fluoropolymer has a lower glass transition temperature than room temperature and is therefore unreliable for mechanical strength in high temperature, and an aliphatic polymer can have poor durability against solvents or oxidation. The publication has no mention of the pore size of the porous polymer solid.
JP-A-11-306858 proposes a solid polyelectrolyte composed of a fluoropolymer matrix containing an imidazolium salt and a lithium salt. It is virtually gel and liable to deformation under external pressure, which can pose a strength problem. A fluoropolymer has a lower glass transition temperature than room temperature and is therefore unreliable for mechanical strength in high temperatures.